Variable resistive devices utilize elements that vary a voltage or current in order to provide an electrical signal that indicates a relationship to a physical position of a contact or wiper on a resistive or conductive element. Because these contacts or wipers are used in a dynamic state they cannot be fixed or restricted in their movement and must have the freedom to slide or move along any length of their respective resistive or conductive paths. These elements or tracks are custom formulated by each manufacturer and will vary in composition and properties. Because the contact and element have the potential for creating constant friction, the contact or wiper must therefore be produced of a material that is electrically, physically, and environmentally compatible with the resistive and/or conductive track when in the presence of an electrically active and physically dynamic system. The contact or wiper must also provide a long useful life, while maintaining uniform positive engagement with the resistive or conductive element, at a specified applied force, and should not encourage or stimulate the growth of polymers or debris, which act as an insulator and which distort the output signal.
Presently the contact or wiper materials used for these variable resistive devices are composed of various solid precious metals, clad or coated metals, or precious metal alloys. These precious metal containing contacts, in a dynamic state and in the presence of electrical activity, act as catalysts to generate polymers and debris which degrade the resistive track output signals. This results in the early termination of accurate performance and useful life.
Initially metal contacts or wipers were used with wirewound resistive or metallic conductive elements, because wirewound elements were the most precise devices. As time evolved great improvements were made in the non-wirewound product area, and they supplanted the wirewound resistive element, but the contact or wiper has always created problems relative to the resistive element because in the presence of an electrical current and dynamic performance, the precious metal components of the metallic contact provide the catalyst to generate polymers and debris, which interfere with the accuracy of the output signal.
Now that reduction in size, improved accuracy, lower voltages, reduced currents, and a reduction in electrical contact resistance are required in modern servo feedback positioning systems, non-metallic contact materials must be considered to obtain the necessary and sorely needed improvements in these performance characteristics and elimination of the polymers and debris.
Also, the primary metal currently used in the precious metal alloy is Palladium. This metal has seen a 1,800% price increase since its introduction for use in this application. The price increase has been largely due to an uncertain supply of this metal.
Also, new environmental laws are being introduced world-wide mandating that automotive components, which are the largest industry using the device described above, be 100% recyclable. The precious metal currently being used cannot be recycled, so that there will be a conflict with this mandate.
Accordingly, the need exists for improvements in electrical contacts and contact assemblies and, particularly, for-improvements in the materials and assemblies employed therefor.